In recent years, from the viewpoint of global environmental protection, to control CO2 emissions, there has been an urgent need to improve the fuel efficiency of automobiles and there has been a demand for weight reduction by reducing the thickness of materials to be used. However, such a reduction in thickness degrades crashworthiness. Since there has also been a requirement to improve safety to ensure the safety of occupants at the time of a vehicular collision, it is essential to increase the strength of materials to be used to achieve reduction in thickness.
Many automobile components for which a steel sheet is used as a material are manufactured by press forming. In general, by increasing the strength of a steel sheet, ductility, stretch flangeability, and the like are degraded and springback is increased. Therefore, formability and shape stability remain problems to be solved. In recent years, it has become possible to predict the amount of springback with high accuracy by CAE (Computer Assisted Engineering). When there is a large variation in material quality, the accuracy of prediction by CAE deteriorates. Therefore, there has been a demand for a high strength steel sheet having, in addition to formability, excellent uniformity of material in which the variation in strength is small.
Currently, development has been actively conducted to achieve both high strength and good formability. For example, Japanese Unexamined Patent Application Publication No. 2007-9322 discloses a high strength hot-rolled steel sheet having excellent ductility, stretch flangeability, and tensile fatigue with a TS of 780 MPa or more, which has a chemical composition including, in percent by mass, 0.06% to 0.15% of C, 1.2% or less of Si, 0.5% to 1.6% of Mn, 0.04% or less of P, 0.005% or less of S, 0.05% or less of Al, 0.03% to 0.20% of Ti, and the balance being Fe and incidental impurities, which has a microstructure including 50% to 90% of a ferrite phase, in terms of volume fraction, and the balance being substantially a bainite phase, the total volume fraction of the ferrite phase and the bainite phase being 95% or more, in which precipitates containing Ti are precipitated in the ferrite phase, and the average diameter of the precipitates is 20 nm or less, and in which 80% or more of the Ti content in the steel is precipitated.
Furthermore, Japanese Unexamined Patent Application Publication No. 2007-302992 discloses a high strength hot-rolled steel sheet having excellent stretch flange formability with a TS of 690 to 850 MPa and a λ of 40% or more, which has a chemical composition including, in percent by mass, 0.015% to 0.06% of C, less than 0.5% of Si, 0.1% to 2.5% of Mn, 0.10% or less of P, 0.01% or less of S, 0.005% to 0.3% of Al, 0.01% or less of N, 0.01% to 0.30% of Ti, 2 to 50 ppm of B, and the balance being Fe and incidental impurities, in which the relationships 0.75<(C %/12)/(Ti %/48)−N %/14−S %/32)<1.25 and 1.0<(Mn %+Bppm/10−Si %) are satisfied, the total area fraction of ferrite and bainitic ferrite phases is 90% or more, and the area fraction of cementite is 5% or less.
Japanese Unexamined Patent Application Publication No. 2002-322541 discloses a high tensile strength hot-rolled steel sheet having high formability and excellent uniformity of material with a TS of 610 to 830 MPa, which contains, in percent by mass, 0.1% or less of C, 0.05% to 0.6% of Mo, and 0.02% to 0.10% of Ti, in which carbides containing Ti and Mo in the range satisfying the atomic ratio Ti/Mo≧0.1 are dispersed and precipitated in the microstructure including a ferrite structure.
Furthermore, Japanese Unexamined Patent Application Publication No. 2009-185361 discloses a high strength hot-rolled steel sheet having excellent uniformity of strength with a small variation in strength with a TS of 540 to 780 MPa, which has a chemical composition including, in percent by mass, 0.05% to 0.12% of C, 0.5% or less of Si, 0.8% to 1.8% of Mn, 0.030% or less of P, 0.01% or less of S, 0.005% to 0.1% of Al, 0.01% or less of N, 0.030% to 0.080% of Ti, and the balance being Fe and incidental impurities, in which the area fraction of a polygonal ferrite phase is 70% or more, and the amount of Ti present in precipitates with a size of less than 20 nm is 50% or more of the value of Ti* calculated by the expression [Ti*=[Ti]−(48/14)×[N]].
However, in the high strength hot-rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 2007-9322, hard ferrite and bainite phases are required to be produced at specified volume fractions. Since the transformation behavior is not constant with respect to the chemical composition of steel, there is a problem that controlling is difficult during air-cooling for promoting the ferrite transformation. In the high strength hot-rolled steel sheet described in each of Japanese Unexamined Patent Application Publication No. 2007-302992 and Japanese Unexamined Patent Application Publication No. 2002-322541, elongation El is low, and it is not necessarily possible to obtain a steel sheet having good stretch flangeability and material stability, which is a problem. In the high strength hot-rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 2009-185361, a TS of 590 MPa or more is obtained by solid-solution strengthening using Mn. However, in solid-solution strengthening, the strengthening ratio relative to the amount of the element added is smaller than that in precipitation strengthening using Ti, and thus cost performance is poor. Furthermore, since the amount of C added is large relative to Ti, formation of hard cementite is unavoidable. Therefore, stretch flangeability is poor, which is also a problem.
It could therefore be helpful to provide a hot-rolled steel sheet having high strength and excellent ductility and stretch flangeability, and having good uniformity of material in which the variation in strength in a coil is small, and a method of manufacturing the same.